At the contact of these kinds of electrical sliding contactors, high-temperature switching arc is generated on switching operation or detaching and or chattering the trolley wire because said contact is switched under high current condition of more than 12 Volts (12 V is almost minimum voltage drawing of arc) and slided under the condition of high electric voltage and current. Under these conditions, harder copper alloys having high conductivity are employed as the contact. Small amounts of various kinds of metals are added to most copper alloys in order to increase their hardness and heat resistance. The metals having outer shells of electrons of d.sup.8 -d.sup.10 such as Fe, Ni, Cu, Co, Zn, Ag and the like which are active for adsorption of hydrogen molecules at high temperature or alkaline earth metals whose oxides are active for adsorption of hydrogen molecules at a high temperature such as Mg, Ca, Cd, Zn and the like are added to the copper alloys.
When such an electrical sliding contactor consisting of these copper alloys is used, metallic powder is formed by wear of the contact and the electric arc is generated at the switching part or the detaching part of the trolly wire. Under these conditions of high temperature, metallic oxides powder formed by wear of the contact and metallic vapor of the contact bring about dehydrogenation of ingredients of the lubricant such as oil, wax and organic metallic soap applied on the surface of the contact, and cause decomposition and vaporization of, for example, polyether base oil or carbomixation of, for example, hydrocarbon base oil.
Copper oxides function as an oxidation catalyst for the base oils and the metallic soap at high temperature. For example, copper oxides accelerate oxidative deterioration of metallic salt of higher fatty acid such as lithium stearate and the Japanese like into varnishlike material. Therefore a lubricative grease which is the most general lubricant such as a grease comprising polyether base oil or hydrocarbon base oil and lithium stearate is not suited for the aforesaid use because the Japanese grease is exhausted during the use and wear of the contact is increased. A grease which comprises an inorganic filler such as bentonite as a substitute for metallic soap of higher fatty acid such as lithium stearate exhibits larger mechanical wear and poorer lubricant effect.
The aforementioned problems appear in a grease composition which is the subject matter of the former application by the present applicant (Japanese Patent Application No. 182397/1989). When the grease composition comprising a mixture of polyether and hydrocarbon base oils is used as a lubricative grease for a sliding switch having copper contact which makes and breaks at 14 V and 180 W, an insulating property of the switch is decreased because the grease is exhausted with a repetition of switching and the metallic powder formed by wear of the contact is adhered on the surface of an insulator in the vicinity of a breaking part of the contact whereat the electric arc is generated. In order to solve these problems, the present applicant has filed an application relating to improvement of wear-resistance of said grease composition when it is applied on the copper contact of the sliding switch (Japanese Patent Application No. 14453/1990), said grease composition being not suited for general usage.
The patent application No. 14453/1990 is based on the fact that, when the contact made of copper alloy in which small amounts of Sn are dispersed homogeneously is employed as a contact of the sliding switch, the grease composition disclosed in the patent Application No. 182397/1989 is not exhausted, wear of the switch is decreased, the metallic powder formed by wear of the switch is finer than that formed by wear of the switch comprising no tin and usability life of the switch is lengthened in spite of repetition of durability test of the switch. It has been found that the same effects as those mentioned above are obtained when a grease comprising .alpha.-olefin base oil and lithium stearate or a grease comprising polyol ester base oil and bentonite is employed in place of said grease composition disclosed in the patent application No. 182397/1989. It has also been found that more amounts of Sn are necessary to obtain the aforesaid effects when Fe is added to copper alloy. At the breaking part whereon arc heat is irradiated, oxides powder of Sn and Cu formed by wear of the contact or vapor thereof in the arc can function as an active catalyst. Based on an assumption that decomposition and deterioration of the grease which are caused by adsorption of hydrogen molecule as previously stated may be prevented effectively by SnO.sub.2 (oxide powder of Sn) the present inventors have found that the aforesaid problems can be solved and expected effects are obtained by blending this compound with the grease composition homogeneously.